Semiconductor fabrication processes are typically conducted with the substrates supported within a chamber under controlled conditions. For many processes, semiconductor substrates (e.g., silicon wafers) are heated inside the process chamber. For example, substrates can be heated by direct physical contact with an internally heated wafer holder or “chuck.” “Susceptors” are wafer supports used in radiantly heated system where the wafer and susceptor absorb radiant heat.
Susceptors are commonly formed by machining graphite into a desired shape and applying silicon carbide (SiC) coatings. Susceptors can be formed in different shapes, but many are circular. Susceptors are often formed with grid patterns on the front face, the surface which faces the wafer. These patterns are normally configured to provide a volume underneath the wafer to allow gas to compress and escape, to minimize wafer sliding when a wafer is placed on a susceptor, and for gas to expand and flow in under the wafer, to minimize wafer sticking when the wafer is lifted from the susceptor. See U.S. Pat. No. 5,403,401.
Current susceptor designs, particularly those designed to accommodate 200 mm, 300 mm or larger wafers, generate large temperature gradients when a cold wafer is loaded onto a hot susceptor. These temperature gradients in the wafer cause the wafer to curl. During this curl, damage to the backside of the wafer can occur through contact with the susceptor. Therefore, minimizing backside damage is highly desirable in the field of wafer processing.
It has been found that the asymmetrically high surface area on the susceptor front side as compared to the flat backside, in combination with the differential coefficient of thermal expansion (CTE) of the graphite core and SiC coating, results in the susceptor bowing or “dishing” into a bowl shape during high temperature susceptor fabrication processes. U.S. Pat. No. 5,403,401 provides the same grid pattern on the susceptor backside, avoiding the front and backside differential and thereby avoiding the dishing phenomenon. However, the concave dish shape has been found advantageous in minimizing wafer curl upon cold wafer drop-off. On the other hand, conventional gridded susceptors, with grids arranged in an X-Y checkered configuration, have been found to cause uneven dishing, which can result in an asymmetric “saddle” shaped susceptor, rather than a “dish” shape. A saddle shaped susceptor causes damage to the backside of a wafer, leaving the wafer with defective sections. Susceptor warping is especially damaging when processing large wafers.
Accordingly, a need exists for better control over susceptor shape, and for reducing backside damage to wafers form wafer curl.